Data transmission speed between racks, between boards, and within a board has recently increased along with a significant improvement of performance of central processing units (CPU) of information processing apparatuses and/or communication apparatuses (e.g., high-end servers), whereas increase of speed in conventional electrical wirings has come close to its limit. Thus, optical interconnect technology enabling higher-speed data transmission has been considered.
In high-speed optical interconnect, a light-emitting element is directly modulated to transmit an optical signal on the transmitting side, and a light-receiving element receives the optical signal and converts it to an electrical signal on the receiving side. However, the light-emitting element driven at a high speed causes waveform deterioration such as eye closing due to relaxation oscillation of the light-emitting element and/or bandwidth limitation, thereby deteriorating transmission quality. To cope with this, preemphasis for correcting (emphasizing or suppressing) in advance the rising portion and the falling portion of a drive signal for the light-emitting element is known.
A preemphasis generating circuit divides an input signal into two, and includes a main-signal amplifying circuit and a current subtraction circuit that delays the input signal. The main-signal amplifying circuit includes a differential pair of transistors, a current source, and a current supplying source. The current subtraction circuit includes a differential pair of transistors that amplifies a differential signal delayed for preemphasis, and obtains a preemphasis component based on the difference between the input signal and the delay component. A preemphasis signal is obtained by the main-signal amplifying circuit and the current subtraction circuit (see, for example, Japanese Laid-Open Patent Publication Nos. 2007-81608 and 2008-219895).
However, the current supplying source of the conventional preemphasis generating circuit supplies a current to the preemphasis generating circuit in addition to the main-signal amplifying circuit. Thus, a transistor included in such a high-power current supplying source requires a high maximum rated current, thereby increasing parasitic capacitance and preventing high-speed operation. The entire power consumption increases if current supplying sources are provided for the main-signal amplifying circuit and the preemphasis generating circuit, respectively.